Medical analysis of the heart muscle has revealed that each normal heart contraction originates from an area in the upper right atrium called the sinus auricular node, and spreads in the form of a depolarization wave through the atrioventricular node, across the heart to the ventricular myocardium. The depolarization wave then spreads through the muscular tissue of the ventricle to cause the ventricle to contract for pumping blood through the arteries.
Thus, although normal contraction of the heart is referred to in common parlance as being a "heartbeat", in actuality the contraction proceeds as a wave which travels across the surface of the heart. In the event that various cells within the heart tissue have been damaged, propagation of the depolarization wave across the heart may be obstructed. Furthermore, in the event that the cells in a specific region of the heart have been damaged, conflicting depolarization waves may be generated by the affected cells which interfere with the normal heart rhythm, a condition known as cardiac arrhythmia.
The surgical treatment of cardiac arrhythmias has been facilitated by an understanding of the mechanisms of arrhythmia gained through a precise description of the structure and function of the cardiac tissues. To this end, advances in medical technology have resulted in development of various devices for investigating electrical activity, and thereby interoperatively identifying the sources of cardiac arrhythmias within a patient.
One such developmental tool is known as a cardiac mapping system comprising an electrode array having a plurality of electrodes arranged in a three-dimensional grid, a plurality of preamplifier units for amplifying signals received from the electrode array, a data acquisition sub-system for performing analog-to-digital conversion of the signals received from the preamplifier units, and an analysis and display processor for displaying individual epicardial waveforms as they propagate across the heart during each contraction.
In operation, the chest cavity of a patient is opened and the electrode array is located over or within the heart muscle. The electrodes detect bioelectric phenomena of the heart muscle at their individual locations across the surface of the heart and in response generate corresponding analog-electrical impulses representative thereof. The analysis and display processor captures and processes the data received from the acquisition sub-system and displays the individual waveforms. The information is typically displayed on a colour monitor as well as remote monitors in the operating room in the form of an isochronal map. Preferably, the data from the electrodes are then stored on an optical disc or other suitable storage apparatus.
It is important that proper functioning of the cardiac mapping system be assessed prior to use on patients since interpretation of results in the operating theatre will determine the diagnosis and hence the procedure to be performed.
A number of prior art systems have been developed for generating signals which simulate various electrophysiological impulses. For example, U.S. Pat. No. 3,323,068 (Woods) discloses an electrocardiogram simulator for generating EKG waveforms of the human heart. The simulator according to this prior art patent generates a single pulse conforming to a standard idealized EKG wave in order to set up or trouble shoot EKG analysis equipment.
Similarly, U.S. Pat. No. 3,469,115 (Cartridge) discloses a cardiac waveform simulator for generating a pulse having a generally triangular shape and a rise time to fall time characteristic closely resembling the pulses of a human cardiac waveform.
U.S. Pat. No. 4,204,261 (Ruszala et al) teaches a complex analog signal generator for generating a complete complex waveform which is divided into a plurality of outputs for testing and calibrating various types of medical equipment such as electrocardiogram displays and blood pressure waveform displays. Related U.S. Pat. No. 4,205,386 (Ruszala et al) teaches an electrocardiographic and blood pressure waveform simulator device for simulating both electrocardiographic and blood pressure waveforms, with the beginning of the blood pressure waveform being delayed from the beginning of the electrocardiographic waveform so that the waves are provided in a time sequence corresponding to waveforms that would ordinarily be supplied by a live patient.
U.S. Pat. No. 4,352,163 (Schultz et al) discloses a vector-cardiogram simulator for generating three distinct waveforms for simulating electrical activity within the human heart along three separate axes. The three generated waveforms are applied to the input of a vector-cardiogram machine for the purpose of calibration and testing.
The above discussed prior art patents all relate to systems for generating analog signals representative of electrophysiological activity in a single dimension with respect to time. A typical display output for such prior art systems would be in the form of a graph depicting electrical amplitude on one axis versus time on the other axis. Thus, such prior art systems provide signals which simulate the electrophysiological characteristics of a heartbeat, but do not provide for simulation of electrophysiological waves in two dimensions with respect to time (i.e. a simulation of the depolarization wave which travels across the heart surface).